This invention relates to a receiver for use in a digital communication system and, in particular, to a receiver with an AGC (Automatic Gain Control) circuit for controlling a gain so as to keep an average amplitude of a demodulator input signal at a constant level.
Among a wide variety of digital communication systems, a US digital cellular system, for example, is known as a digital communication system using a modulated signal modulated in a QPSK (.pi./4 quadrature phase shift keying) modulation system. In a receiver for use in the digital communication system of the type described, a demodulator input signal supplied to a demodulator must have an average amplitude kept at a constant level and be free from distortion. Otherwise, a reception input signal can not be correctly demodulated. It is therefore required to keep the average amplitude of the demodulator input signal at a constant level even if a reception input level or an input field strength is fluctuated due to fading or the like. To this end, the receiver of the type described comprises an AGC circuit including a low noise amplifier connected to a reception antenna and an AGC amplifier located at an input side of the demodulator.
In such a conventional receiver with an AGC circuit, however, the AGC amplifier is saturated when the input field strength is increased. This is because the low noise amplifier has a high gain. In this event, distortion is caused in the demodulator input signal. As a consequence, the demodulator can not normally operate so that a demodulated signal inevitably contains an error.
Taking an economical efficiency into account, the AGC amplifier is allowed to have a dynamic range as wide as about 70 dB at most. Thus, the AGC amplifier is normally operable at the input field strength between -115 dBm and -45 dBm. When the input field strength is higher than -45 dBm beyond the above-mentioned range of the AGC amplifier, the average amplitude is fluctuated. In order to avoid fluctuation of the average amplitude, the AGC amplifier must have a high saturation level and a wide dynamic range. This inevitably increases the scale of the AGC circuit and the amount of electric current supplied thereto. It is therefore difficult to efficiently realize a high saturation level and a wide dynamic range.